1. Field of the Invention
The present invention relates to completion/reservoir technology, and more particularly to a method of geometric evaluation of hydraulic fractures for a multi-well pad.
2. Description of Background Art
Over the years, the research on reservoir technology focuses on maximizing the value of ultra-tight resources, sometimes referred to as shales or unconventional resources. Ultra-tight resources, such as the Bakken, have very low permeability compared to conventional resources. They are often stimulated using hydraulic fracturing techniques to enhance production and often employ ultra-long horizontal wells to commercialize the resource. However, even with these technological enhancements, these resources can be economically marginal and often only recover 5-15% of the original oil in place under primary depletion. Therefore, optimizing the development of these ultra-tight resources by evaluating geometry of hydraulic fracture so as to optimize the well spacing and completions is critical. In addition to improving economics with optimized well spacing and completions, increasing certainty around hydraulic fracture geometry will also enable increased certainty around matrix permeability since these two parameters are often integrally linked in production analysis. Improved understanding of matrix permeability will lead to a better predict of decline curves, and thus, ultimate recovery estimates and reserves estimates. Moreover, with the increase in demand of maximizing the value from the unconventional reservoirs, enhanced oil recovery (EOR) technologies are becoming increasingly important. One of the key aspects of nearly all EOR technologies is well to well communication. An improved understanding of hydraulic fracture geometry will also enable better evaluation of the EOR potential in unconventional reservoirs.
Although the importance of understanding hydraulic fracture geometry has been recognized in industry for well over a decade, a low-cost, technically robust technology, which can map hydraulic fractures has yet to be commercialized. Hydraulic fracturing has been used for decades to enhance the producibility of tight-gas reservoirs. The fundamentals of fluid transport in fractures, matrix leakoff, and fracture mechanics during fracture propagation have been well-studied, leading to the development of pseudo-3D and planar 3D fracture propagation simulation models, as well as bottomhole treatment pressure analysis tools. These tools have been widely used for estimating fracture lengths and drainage boundaries in hydraulically fractured tight-gas reservoirs. However, despite the wealth of knowledge in tight-gas reservoirs and studies on hydraulic fracture propagation dating back to when Sneddon (1946) developed one of the first fracture propagation models, understanding the fracturing process in unconventional reservoirs is still in its infancy. Shale reservoirs are complex and heterogeneous. Moreover, they often contain natural fractures, faults, and other planes of weakness, which can complicate fracture propagation. The interaction between hydraulic and natural fractures can lead to reactivation of natural fractures and complex fracture growth. Although there have been recent attempts to model complex fracture propagation, the mechanics of network growth is not fully understood, and reservoir characterization and simulation in three dimensions remains challenging. This has limited the applicability of fracture models in ultra-tight, complex plays.
In conventional oil fields, there are many methods used for attempting to evaluate hydraulic fracture geometry and optimize well spacing. One of the most common methods which has been widely adopted is to use subsurface or surface micro-seismic arrays to monitor seismic events during the hydraulic fracturing process. Ideally, this would provide insight into the dimensions of hydraulic fractures, helping to determine the optimal well-to-well spacing. However, this technology is costly and is often questionable for a number of reasons. First, and foremost, it is often accepted that microseismic predominantly identifies shear events, which may or may not be associated with the growth of hydraulic fractures. Microseismic events are linked with the creation and dilation of hydraulic fractures but do not necessarily only occur where the fracture fluid or even proppants are placed. The stress state in the rocks adjacent to the hydraulic fracture is altered from its initial state and hence there are plenty of possible explanations for microseismic events, for example by reactivating pre-existing planes of weakness or micro fractures within the surrounding rock which are not at all hydraulically connected to the well. Therefore there is a huge uncertainty on the hydraulic fracture geometry. A second challenge with microseismic is that it requires knowledge of the subsurface, particularly wave velocities in the media, which are often unknown and have high uncertainty. Finally, the processing methods themselves are often brought into question, as many service companies who provide this technique use veiled algorithms and openly admit the uncertainty in these processing methods.
Another technology which has been used to evaluate hydraulic fracture geometry is downspacing tests, where varying well-to-well spacings are chosen for different pads and production is compared at different spacings to assess which spacing is optimal. This technique is expensive and time consuming and often gives a highly uncertain answer, requiring this procedure to be repeated many times, in a cost inefficient manner, to increase accuracy in the result. This procedure, which often ends up with under drilling and over drilling numerous pads, can significantly reduce the value of the resource due to inefficient development.
There are other alternative technologies for mapping hydraulic fractures currently being explored, but many of these technologies provide only qualitative information or require expensive data acquisition tools.
To date, no methods for evaluating hydraulic fracture geometry and optimizing the well spacing with less cost, more accurate results, and much fewer wells and inefficiently developed pads compared with the above mentioned conventional methods, have been successfully deployed in ultra-tight oil resources. Therefore, there is an industry-wide need for a method for evaluating hydraulic fracture geometry and optimizing well spacing for a multi-well pad in order to better understand optimal well-to-well spacing, so as to maximize the value of ultra-tight resources with less cost and higher certainty.